Intel 8XC196MC manual Understanding A/D Conversion Errors, Using Mixed Analog and Digital Inputs

ANALOG-TO-DIGITAL (A/D) CONVERTER

ANGND should be within about ± 50 mV of VSS. VREF should be well regulated and used only

for the A/D converter. The VREF supply can be between 4.5 and 5.5 volts and must be able to source approximately 5 mA (see the datasheet for actual specifications). VREF should be approx-

imately the same voltage as VCC. VREF and VCC should power up at the same time, to avoid poten- tial latch-up conditions on VREF. Large negative current spikes on the ANGND pin relative to VSS

may cause the analog circuitry to latch up. This is an additional reason to follow careful ground- ing practice.

The analog reference voltage (VREF) is the positive supply to which all A/D conversions are com- pared. It is also the supply to port 0 (and port 1 for the MC and MD) if the A/D converter is not being used. If high accuracy is not required, VREF can be tied to VCC. If accuracy is important, VREF must be very stable. One way to accomplish this is through the use of a precision power sup- ply or a separate voltage regulator (usually an IC). These devices must be referenced to ANGND, not to VSS, to ensure that VREF tracks ANGND and not VSS.

12.6.1.4Using Mixed Analog and Digital Inputs

Port 0 (and port 1 for the MC and MD) may be used for both analog and digital input signals at the same time. However, reading the port may inject some noise into the analog circuitry. For this reason, make certain that an analog conversion is not in progress when the port is read. Refer to Chapter 6, “I/O Ports,” for information about using the port as digital inputs.

12.6.2 Understanding A/D Conversion Errors

The conversion result is the ratio of the input voltage to the reference voltage.

RESULT (8-bit) = 255 ×	VIN – ANGND		RESULT (10-bit) = 1023 ×	VIN – ANGND
	V-----RE-------F----	–----ANGND----------------------		V-----RE-------F----	–----ANGND----------------------

This ratio produces a stair-stepped transfer function when the output code is plotted versus input voltage. The resulting digital codes can be taken as simple ratiometric information, or they pro- vide information about absolute voltages or relative voltage changes on the inputs.

The more demanding the application, the more important it is to fully understand the converter’s operation. For simple applications, knowing the absolute error of the converter is sufficient. However, closing a servo-loop with analog inputs requires a detailed understanding of an A/D converter’s operation and errors.

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